Half.com Charged me for book I sold and shipped then sent debt collector after me Internet

I sold a book on half.com and mailed the book 4 business days after selling it. The half.com policy apparently says you have to mail the book 3 business days after selling. Well, the book I sold got to the buyer late and the buyer filed a claim against me. The buyer ended up getting the book and getting a refund for it. So, I ended up with no book and minus over $100 for the price of refunding the seller. I tried and tried to get in contact with half.com and they have NO phone number. You can only contact them through email! So, I sent my bank after them and finally got my money credited back to my account from my bank. Now, a year later, half.com has come back saying they want the $100 from where I sold the book and the money got credited to the buyer. The buyer got his book and they still are trying (a year later) to debit my account??? So, they threatened to send a debt collector if I did not respond within 7 days. I responded within 7 days and did not get a response from them for 2 weeks. Then, today I get a note in the mail from a debt collector. This company is bad news. They do bad business and have terrible communication skills. I give them an F- and I will spend as much time as I can telling everyone I know how much I hate half.com and would never recommend it to anyone. In the end, even if they somehow get my $100 for the book I mailed off, I will cost them way more than that by telling everyone about their horrible customer service and shameful business practices. This website should know that word of mouth is very powerful and I plan on using it to spread how horrible their company was to me.